(a) Field of the Invention
This invention is concerned with die assemblies, more especially with die assemblies for use with a screw extruder to provide an insulating plastic covering on a core; the invention is also concerned with a method of extruding an insulating plastic covering around an advancing core.
(b) Description of Prior Art
Electrical conductor wires in, for example, power cables and communication cables are electrically insulated by extruding a molten plastic around the wire as it advances through a cross-head die assembly and solidifying the plastic on the wire.
The insulating covering, particularly in a communication cable conductor wire, may be cellular or solid. In the case of cellular coverings the molten plastic material contains gaseous material which expands as bubbles in the molten material as the coated wire emerges from the die assembly. The degree of expansion is controlled by the cooling to solidification, which is suitably achieved by advancing the coated wire through a water trough which moves in response to signals from a capacitance monitor.
Similarly an insulating sleeve may be formed around a bundle of insulated conductors by extruding the molten plastic around the advancing bundle. In the case of communication cables the bundle suitably has a shield of aluminum wrapped around it, the shield conveniently having a synthetic coating of a carboxylated polymer which adheres readily to the molten plastic extruded around it.
Insulating coverings particularly in communication cables are required to meet exacting standards, particularly with regard to their thickness. The thickness of the covering and the degree of cellularity in the case of cellular insulation, at any point along the insulated covering determines the electrical characteristics, for example, the dielectric properties of the insulated conductor. Variations in the electrical characteristics along the insulated conductor can result in cross-talk in a telephone cable and this is difficult to eliminate.
It is generally desirable and necessary to form an insulating covering on a conductor particularly for a communication cable, which is uniform along the insulated conductor.
Generally the concentricity, roundness and uniformity of the insulating covering must be carefully controlled.
In many cases it has, in addition, been a practice to provide a thicker insulating covering than actually required so that at least a minimum insulation will be provided over the entire wire surface, however, this is uneconomic.
In the conventionally employed cross-head die assemblies molten plastic insulating material is extruded from a screw extruder into the die assembly, in a direction transverse to the direction of travel of the core advancing through the die assembly and onto the advancing core. This necessitates that the stream of molten plastic undergo at least one 90.degree. change in direction in its flow path, which results in flow imbalance conditions in the molten plastic. In particular a differential of pressure or flow is set up between the molten plastic in the outside regions of the bending stream and the molten plastic in the inside region of the bending stream.
The flow imbalances in the stream as it flows onto an advancing core cause eccentricity between the insulating covering and the core, and lack of roundness of the outer circumference of the extruded covering. This results in cross-sectional and longitudinal variations in the thickness of the formed covering, which, as indicated above, is unacceptable.
In U.S. Pat. No. 3,860,686, Daryl Lester Myers, issued Jan. 14, 1975, a multi-path cross-head die assembly has been proposed. In the die assembly of Myers the molten plastic is subjected to a number of changes of flow in which the stream is divided and the direction of flow is changed. Unfortunately the Myers die assembly is impractical, particularly in that it is almost impossible to adequately clean the inaccessible, multiplicity flow channels which define a tortuous path for molten material in the assembly. It is necessary to thoroughly clean die assemblies, for example, to remove deposits of charred plastic which may collect, particularly in corners, and when the plastic being extruded is to be changed.
Thus the Myers die assembly does not provide a practical solution to the problem and does not appear to have been exploited commercially.
It is an object of the present invention to provide a die assembly which overcomes the prior difficulties and enables the formation of an insulating covering of an acceptable high standard of uniformity on an advancing core while at the same time the surfaces of the assembly exposed to molten plastic are accessible and can be cleaned without difficulty in a simple cleaning operation.
It is a further object of the invention to provide die assemblies which can be employed to form insulating coverings on electrical conductor wires, as well as insulating sheaths around bundles of insulated conductor wires.
It is a further object of the invention to provide an extrusion die for use in the extrusion of molten insulating plastic around an advancing core.
It is a further object of the invention to provide a core guide for guiding an advancing core through an extrusion die.
It is yet another object of the invention to provide a method of extruding an insulating plastic covering around an advancing core, particularly an advancing electrical conductor wire.